A hybrid heat treatment furnace including a gas combustion furnace and an electric furnace has been developed for the purpose of satisfying as much as possible the requirements of energy saving and high precision control. The hybrid heat treatment furnace operates as follows.
(I) The hybrid heat treatment furnace operates the energetically favorable gas combustion furnace when raising a temperature from a lower temperature to a higher temperature.
(II) The hybrid heat treatment furnace operates the more responsive electric furnace when maintaining a constant temperature with high precision after raising the temperature.
FIG. 5 is a diagram illustrating an example of a temperature control system in a hybrid heat treatment furnace. In the example of FIG. 5, an electric heater 101, a gas heater 102, and a temperature sensor 103 are installed inside a heat treatment furnace 100. The temperature sensor 103 measures a temperature PV (process variable) of an air heated by the electric heater 101 or the gas heater 102. A temperature controller 104 calculates a manipulation variable MV so that the temperature PV matches a setting value SP. A power conditioner 105 supplies an electric power corresponding to the manipulation variable MV to the electric heater 101. A gas flow rate regulator 106 supplies a gas at a flow rate corresponding to the manipulation variable MV to the gas heater 102. The temperature controller 104 controls a temperature in the heat treatment furnace 100.
As described above, when raising the temperature of the heat treatment furnace 100 from a lower temperature to a higher temperature, the hybrid heat treatment furnace operates the energetically favorable gas combustion furnace with the use of the gas flow rate regulator 106 and the gas heater 102. On the other hand, when maintaining the temperature of the heat treatment furnace 100 at a constant value with high precision after raising the temperature, the hybrid heat treatment furnace operates the more responsive electric furnace with the use of the power conditioner 105 and the electric heater 101 although the electric furnace is inferior to the gas combustion furnace in terms of energy efficiency.
PTL 1 has proposed a controlling device which switches from a gas mode to an electric mode quickly and surely in the hybrid heat treatment furnace described above.
In addition to the hybrid heat treatment furnace, various improvements to the temperature control system of a combustion based furnace have been tested. In a temperature control system for a combustion based furnace, in order to control the temperature the flow rate of a fuel such as gas has to be controlled. For example, a control valve capable of adjusting the flow rate is employed as the gas flow rate regulator. A motor position for operating the valve is represented by the manipulation variable MV that is an output of a proportional integral derivative (PID) calculation.
However, since the motor is constrained by a movable body whose operating speed is limited, the motor position does not always actually follow the manipulation variable MV calculated by the PID calculation. In contrast, in an electric heater, an electric current value follows the manipulation variable MV without substantial delay. Therefore, in order to confirm that the motor position follows the manipulation variable MV, a motor feedback value MFB is incorporated into a controller. Although the manipulation variable MV and the motor feedback value MFB are theoretically the same physical quantity, the manipulation variable MV and the motor feedback value MFB are distinguished from each other in monitoring assuming that the manipulation variable MV and the motor feedback value MFB do not match each other.
Further, without being limited to a temperature control system, the use of feedback can be applied in other systems. In a control valve (see PTL 2) used in the flow rate control system in a petrochemical plant, likewise, in order to confirm that a valve stem position follows the manipulation variable MV, a position feedback may be incorporated into a controller.